[TowerTalk] Elevated vertical

[K8RI]

On 12/25/2011 4:10 PM, Jim Brown wrote:
> On 12/24/2011 10:07 AM, john at kk9a.com wrote:
>> The measured impedance will change depending on the length of the coax.  It
>> should match the antenna feedpoint impedance at 1/2 wavelength multiples.
>> The SWR should not change with coax length, except for the change due to
>> loss.
> "except for the change due to loss can be a VERY important qualifier."
> I recently used N6BV's TLW software (free with the ARRL Antenna Book) to
> do the math for 4.5 wavelengths at 21 MHz for some 75 ohm line I was
> using. 50 ohms at the antenna looks like 55 ohms at the transmitter end
> of the line, due to the line loss.

Unless I'm missing something line loss always makes the SWR appear 
*less*.  Line length can bring up nulls and nodes when the antenna is 
reactive. If the antenna is 50 ohms X= 0 then regardless of line length 
the it should show 50 ohms at any length.  Only with a reactive load 
should a line make the impedance different at the source. One way of 
checking coax is the SWR, but not in the conventional sense. If the SWR 
gets *better* over time you know the coax is deteriorating.

73

Roger (K8RI)